The use of ink jet printers for printing information on a recording media is well established. Ink jet printers may be grouped into “continuous” inkjet printers that use continuous streams of fluid droplets and “drop-on-demand” inkjet printers that emit droplets only when corresponding information is to be printed. Drop-on-demand inkjet printers have become the predominant type of printer for use in home computing systems, while continuous inkjet systems find major application in industrial and professional environments.
Continuous inkjet printers typically have a print head that incorporates a supply line or system for ink fluid and a nozzle plate with one or more ink nozzles fed by the ink fluid supply. A gutter assembly is positioned downstream from the nozzle plate in the flight path of ink droplets to be guttered. The gutter assembly catches ink droplets that are not needed for printing on the recording medium.
In order to create the ink droplets, a drop generator is associated with the print head. The drop generator influences, by any of a variety of mechanisms discussed in the art, the fluid stream within and just beyond the print head. This is done at a frequency that forces thread-like streams of ink, which are initially ejected from the nozzles, to break into a series of ink droplets in the vicinity of the nozzle plate. A charge electrode is positioned along the flight path of the ink droplets. The charge electrode selectively charges the ink droplets as the droplets break off from the jet. One or more deflection plates positioned downstream from the charge electrodes deflect a charged ink droplet either into the gutter or onto the recording media. For example, the droplets to be guttered are charged and hence deflected into the gutter assembly and those intended to print on the media are not charged and hence not deflected. In some systems, the arrangement is reversed, and the uncharged droplets are guttered, while the charged ones ultimately are printed.
Ink droplet misregistration at the media surface is a problem experienced by continuous ink jet printers. Interactions between droplets as they are propelled along a flight pathtowards the recording surface can cause ink droplet misregistration. One cause for droplet interaction is the aerodynamic drag on droplets. Unless the air velocity matches the drop velocity, local airflow around each drop is affected by the passage of the drop and this will affect the dynamics of trailing drops. Such aerodynamic interactions influence the relative spacing between droplets because they either increase or decrease the velocity of the droplets. As a result, some ink droplets reach the media early while others reach the media late. Drops may even merge in flight. The trailing drops may also experience lateral forces when following a drop on a different deflected trajectory. The overall effect is that the aerodynamic interaction, also called the aerodynamic drag, causes relatively poor printing quality due to droplet misplacement on the media.
In multinozzle print heads aerodynamic drag creates the additional problem of variation in droplet velocity from fluid droplet stream to fluid droplet stream, resulting in further inaccuracies in droplet placement on the media, and consequent poor printing quality.
To address the aerodynamic interaction problem, the prior art utilizes a gas stream, such as air, to compensate for aerodynamic drag on the ink droplets. The air flows collinearly with the stream of ink droplets and reduces the aerodynamic effect. The inkjet nozzle is generally mounted to eject the droplets into the center of the air stream. In an extension of this approach, laminar airflow has also been applied to multinozzle heads. This is generally done by using a single row of nozzles.
The prior art is generally characterized by the placement of a single nozzle centrally in the highest velocity zone of the laminar airflow column. This is done to minimize any forces that may deviate the flight path of the droplets laterally. Laminar flow systems for single rows of multiple inkjet nozzles have also been described in the prior art, the nozzles again being placed centrally in the highest velocity zone of the laminar airflow column. While multirow multinozzle continuous inkjet systems have indeed been proposed, they have not seen the benefit of laminar airflow, due to the above anticipated negative consequences of droplet placement anywhere but in the uniform highest airflow velocity area of the system where the airflow velocity profile is suitably flat. As a result, the inkjet printer designs suggested for multirow multinozzle systems are subject to serious droplet misregistration problems.